Intended Applications:
In the field of ion implantation of semiconductors for transistor fabrication, there is a class of implants which is known as ‘parametric’ (sometimes also characterized as ‘medium current implants’) which uses ion doses typically in the range from 2×1011 to 1×1014 ions per sq. cm. These implants are generally employed for making fine adjustments in control of transistor parameters—such as for controlling the threshold voltage of the transistors, or for a variety of counter-doping purposes, or for controlling the gradient of doping with precision.
The term ‘parametric’ describes the role of these implants in controlling the transistor's operating parameters; in comparison, the term ‘medium current’ reflects the fact that these implants can be performed at high commercial throughputs using relatively low dose beam currents, which typically are below the current level at which space-charge forces would dominate the forces characterizing the ion optics of the implanter system. Medium current implants are typically performed with scanned ion beams. The scanning technique provides a convenient means of controlling the uniformity of the implant.
The incident angle with which the ions impinge on the silicon substrate is of great importance, for a number of reasons, including the following.
a) There is a phenomenon known as ‘channeling’, whereby the implanted ions penetrate the surface of the substrate more deeply if they align with the crystal axes or planes. This occurs because the fraction of the ions which miss initiating a nuclear scattering event at the first plane of encountered substrate atoms will also miss a large number of subsequent planes of atoms, as the deeper planes of atoms lie in almost perfect alignment below the top layer; and it is quite possible for the ions to be guided down a channel between the aligned planes of atoms in the substrate. Also, it has been found that under certain circumstances, the number of nuclear scattering events which are initiated per incident ion can vary by more than 30% in response to a change in the incident angle of far less than 1 degree.
b) Transistor sizes continue to become ever-smaller. The International Technology Roadmap for Semiconductors, published by International Sematech at http://public.itrs.net, predicts the rate at which the critical dimensions of transistors will continue to shrink. As a result of this dimensional shrinkage, the aspect ratio of the openings in the photolithographic masks becomes greater. Also, an increasing fraction of all implants is performed with the ion beam at zero degrees to the normal to the wafer surface, to prevent poorly controlled shadowing of areas of the regions to be doped by the walls of the openings in the mask. Changes in the incident beam angle thus cause many undesired variations by shadowing and dopant placement in the process parameters, whose significance and magnitude is increased by the ever-shrinking dimensions of transistor size.
c) Certain processes exploit the ability of implantation equipment to vary the incident angle of the ions to a substrate in order to implant the ions (through an opening) beneath another feature on the substrate as a buried dopant. However, small variations in incident angle can cause significant variations in the alignment of the buried dopant.
Therefore, for these reasons (and many others), it is today very important to eliminate as much variation as possible in incident ion angle from wafer to wafer, and particularly between different sites on the same wafer; as well as to control these incident angles with a precision of better than about 0.1 degrees.
It will be noted that in the current generation of ion implanters, prior to reaching the wafer, the ion beam typically passes through a focusing lens or similar device (usually a dipole magnet) whose focus coincides with the center of the beam scanning device, thereby rendering the beam trajectories approximately parallel. Many such devices are conventionally known; and these are commonly and collectively referred to herein as ‘Coarse Collimators’.
Serial Implantation Systems:
In conventionally known serial implantation systems, a wafer (or other substrate) is mechanically passed multiple times at constant velocity through a scanned ribbon beam. On each pass, a constant velocity is maintained until each edge of the wafer has substantially cleared the entirety of the ribbon beam. Subsequently, the wafer velocity is rapidly decelerated to zero (a complete stop); and then accelerated to a constant velocity in the opposite direction for another complete pass of the wafer through the ribbon beam. In this manner, the wafer is passed repeatedly multiple times through the beam until the desired full dose of ions has been implanted within the substance of the wafer.
Within such serial implantation systems, the variation of the implant angle remains a variable process parameter requiring ever-tighter control; and some implantation apparatus and systems therefore incorporate devices which use some mode of adjusting the average angle and direction of the beam with respect to the wafer surface. This effort, however, tends to be a single adjustment covering an averaged beam direction; and there remains still a substantial residual individual trajectory variation in beam direction across the locus of the scanned beam within these systems. Also, the adjustment typically combines the system's averaged variation with random changes from day to day in the beam setup conditions. In view of these markedly different and dissimilar circumstances, it would be desirable to measure and compensate for these variations across the full width of the implantation region for the wafer.
Conventionally Used Tools:
In the field of processing substrate materials with ion beams, various techniques have been developed for scanning ion beams and then passing the scanned beam through correcting devices to render the beams more parallel. See for example, U.S. Pat. Nos. 4,276,477; 4,745,281; 4,922,106; and 5,091,655 which describe techniques for parallel-scanning ion beams, and U.S. Pat. No. 4,980,562, which describes a technique for controlling the uniformity of the implant by modifying the shape of the waveform used to scan the beam. The entirety of U.S. Pat. Nos. 4,276,477; 4,745,281; 4,922,106; 4,980,562; and 5,091,655 individually is expressly incorporated by reference herein.
Techniques have also been developed for producing large, approximately parallel, ion beams as well as for also controlling the uniformity of these beams. These techniques are summarized by White et al., [“The Control of Uniformity in Parallel Ribbon Ion Beams up to 24 inches in Size”, in Applications of Accelerators in Research and Industry, AIP conference proceedings, vol. 475, 1998, p. 830].
As used herein, the phrase ‘controlling the uniformity’ means that the current density of charged particles along the long width dimension (the x-axis) of the beam is caused to possess and exist in a desired profile. The density profile itself may be uniform (consistent or homogeneous), or may vary in a chosen pattern or predetermined manner (such as a left-to-right linear ramp).
In contrast, the phrase ‘controlling the parallelism’ means that the individual trajectories of the charged particles constituting the ion beam as a whole have a common directional angle and maintain a substantially similar spatial distance between them as they travel. Accordingly, the parameter of ‘uniformity’ for an ion beam should not be confused with, or be misunderstood as, or be misconstrued as either being the functional or definitional equivalent of or being similar to the parameter of ‘parallelism’ for a traveling ion beam.
Moreover, it is not unusual for one implantation system to attempt to control both parameters—the uniformity and the parallelism—of a traveling ion beam concurrently. For example, the ion implantation system disclosed by U.S. Pat. No. 5,350,926 teaches the use of magnets for analyzing, shaping and rendering more parallel the ion trajectories of a beam; and concurrently, but separately, shows the use of particular multipole elements (integrated or distinct) to control the charged particle uniformity (current density) of the ions in the beam.
Another conventionally known approach, the use of algorithms, should also be properly recognized and understood for what it truly is. Algorithms for adjusting multipoles to achieve greater ion beam uniformity have been developed (as exemplified by those of Diamond Semiconductor Group Inc.); and such algorithms have been used in a variety of commercially manufactured ion implantation products. In one commercial version sold by Mitsui Engineering and Shipbuilding, a discrete multipole structure is present which is formed as a rectangular array of iron pole pieces mounted on a yoke and which surrounds a continuous ion beam via a central rectangular aperture. Each pole piece has a separate wire coil wound around it. When energized, the resulting magnetic field in the central rectangular aperture (through which the continuous ion beam passes) contains spatially varying dipole fields and causes a local deflection of the trajectories for the charged particles passing through it. Subsequently, at a processing stage (or plane of atoms) further downstream in the system, these trajectory deflections produce a well recognized and characteristic variation in current density—in which a one zone or region of the traveling ion beam exhibits a decrease in current density (i.e., uniformity) while a neighboring zone or region of the beam exhibits the converse effect, an increase in current density. See for example, the systems described by U.S. Pat. No. 5,834,786, and the White et al. publication cited above.
In the above described application using method and apparatus for unscanned (ie., continuous) ribbon beams, the parallelism of the ion beam is deliberately degraded (by about +/−0.5 degrees) in order to achieve better uniformity of charged particles. It would theoretically be possible to reverse the tradeoff and achieve a very high degree of beam parallelism at the expense of uniform current density, but the individual parameters of beam parallelism and current uniformity cannot in general be simultaneously optimized. This is a marked disadvantage and obstacle for many ion implantation applications which require both a high degree of particle trajectory parallelism and excellent uniformity of current density.
It will be appreciated also that the commonly available scientific and patent literature provides an abundance of information concerning the marked differences between parallelism and uniformity as well as the tradeoff accommodations which must be made between them, in favor of one parameter over the other. Merely illustrative and representative of these printed publications are the following, each of which is expressly incorporated by reference herein: Berrian et al., U.S. Pat. Nos. 4,922,106 and 4,980,562; Enge, U.S. Pat. Nos. 4,276,477 and 4,745,281; White, U.S. Pat. No. 5,125,575, White et al., U.S. Pat. No. 5,350,926; White et al., U.S. Pat. No. 5,834,786; Iwasawa, U.S. Pat. No. 6,313,474; Aoki, U.S. Pat. No. 6,160,262; and Isobe, U.S. Pat. No. 5,180,918.
For these reasons, were a new development to be made which would allow the parameters of parallelism and uniformity to be concurrently optimized for a scanned ion beam, it would be seen as solving a long-recognized problem and as being of major benefit to practitioners working in the technical field today. Moreover, if such a new development could also provide for two entirely independent controls—which would allow the parallelism of a scanned ion beam to be controlled separately and independently from the beam's current uniformity, this improvement would be acknowledged as being an unexpected advance and as being of unique benefit and advantage for the users of ion implantation devices and systems generally.